Physical processes inside a living cell or tissue, including binding and transport, are of interest in understanding and controlling cell functions, such as protein transport and signaling, energy generation and utilization, differentiation, growth, replication and apoptosis. These functions are important to the detection and treatment of disease, such as cancer. Modern techniques for measuring and monitoring temporal fluctuations in fluorescent particle spatial distributions and single particle trajectories (SPT) provide data to elucidate those physical processes.
Classical fluorescence correlation spectroscopy (FCS) measures fluctuations in fluorescence intensity in a small detection volume to infer molecular properties from governing continuum reaction-diffusion-convection equations, for example. Whereas classical FCS is performed at a single point with a photomultiplier tube (PMT) or Avalanche Photodiode (APD), the relatively recent advent of laser-scanning microscopy and electron multiplying charge coupled device (EM-CCD) cameras has enabled spatially-resolved FCS to be performed in living cells and tissues using confocal microscopy, thus providing a rich source of spatiotemporal information upon which to base biophysical models of biomolecular dynamics
Quantitative measurement and tracking of the trajectories of single particles in living cells and tissues is increasingly common and provides a rich source of information about the physical environments and modes of transport of biological structures. A common metric for inferring the physical mode of particle motion is the mean-squared displacement (MSD) of the particle over time, which takes distinct functional forms depending on the type of motion that generated the trajectory.